1. Field of the Invention
The invention relates to a machine having a rotor which is mounted such that it can rotate about a rotation axis, and which has a hot rotor outer housing which is attached to axial rotor shaft parts and has a superconducting winding which surrounds a cold winding former. The winding former is held within the rotor outer housing by a structure formed at one end of the rotor to transmit a torque between the winding former and the associated rotor shaft part. Also provided is cooling and thermal insulation of the superconducting winding. A corresponding machine is disclosed in DE 23 26 016 B2.
2. Description of the Related Art
Electrical machines, in particular generators or motors, generally have a rotating field winding and a stationary stator winding. In this case, the current density and hence the specific rating of the machine, that is to say the volt-amperes per kilogram of its own weight, can be increased, and the efficiency of the machine can be improved as well by the use of cryogenic and, in particular, superconducting conductors.
Cryogenic windings for electrical machines generally need to be thermally insulated from the environment and to be kept at the required low temperature by a cooling system. Effective thermal insulation can in this case be achieved only if the cryogenic parts of the machine are separated to as great an extent as possible from the hot outer area by hard vacuum with a residual gas pressure of in general less than 10-3 mbar and if the connecting parts between these cryogenic parts and the hot outer area transmit as little heat as possible.
Two variants in particular are known for vacuum insulation of rotors with cryogenic rotor windings and hot stator windings: In a first embodiment, the rotor has a hot outer housing and a closed vacuum area which rotates with it. The vacuum area should in this case surround the cryogenic area on all sides (see, for example, “Siemens Forsch. u. Entwickl.-Ber”, Volume 5, 1976, No. 1, pages 10 to 16). However, heat is transmitted in an undesirable manner to the cryogenic parts via supports which extend through the vacuum area. In a second embodiment, the essentially cold rotor rotates in a hard vacuum. In this case, the outer boundary of the hard vacuum area is defined by the internal hole in the stator. However, an arrangement such as this requires shaft seals which are proof against the hard vacuum, between the rotor and the stator (see, for example, DE 27 53 461 A1).
The first-mentioned embodiment is implemented in the machine that is described in the DE 23 26 016 B2 document cited initially. The superconducting winding on its rotor is located in the interior of a rotor cryostat which, together with closed shafts that are fitted, forms an outer housing for the rotor. Owing to the use of traditional superconductor material for the conductors of the winding, helium cooling is provided, with an operating temperature of around 4 K. In contrast, the outer contour of the rotor outer housing is at approximately room temperature, and even above this temperature during operation. The useful torque of the machine is produced in the rotor winding. This rotor winding is arranged in a cold winding former, which is itself suspended or held in an insulated manner in the rotor outer housing which acts as a cryostat. In this case, this suspension or retention on the drive end of the rotor must be sufficiently stable to transmit the torque from the cold winding former to a shaft part on the drive end. An appropriate, rigid connecting device for torque transmission must therefore be designed in a relatively solid form, and must be connected to the winding former and to the drive-end shaft part such that power can be transmitted. At the same time, this connecting device provides the drive-end centering for the cold winding former. Virtually no torque is emitted at the opposite rotor end, which is also referred to as the non-drive or operating end because important connections such as a coolant supply are provided on it for operation of the machine. Thus, only the functions of centering and thermal insulation need essentially be carried out here. Since, however, with the transition from room temperature to the operating temperature, the axial length of the winding former is reduced by at least a few millimeters relative to the corresponding extent of the rotor outer housing, the operating-end suspension also has to carry out the function of appropriate length compensation. In the machine which is disclosed in the initially cited DE 23 26 016 B2 document, radially running connecting elements in the form of disks are therefore provided between the rotor outer housing and the winding former and allow appropriate bending in the axial direction, for expansion compensation. Heat is also introduced via these connecting elements into the cryogenic area of the winding former.
In a further rotor, which is disclosed in DE 27 17 580 A1, for an electrical machine having a superconducting field winding, a corresponding radially extending connecting element is provided between a rotor outer housing and a winding former which, although it allows axial deformation, leads, however, to undesirable introduction of heat into the cryogenic area of the machine.
In addition to metallic superconductor materials such as NbTi or Nb3Sn which have been known for a long time and as are used in the machines mentioned above, metal-oxide superconducting materials have also been known since 1987, and have critical temperatures above 77 K. Attempts have been made to produce superconducting windings for machines based on conductors using such high-Tc superconductor materials, which are also referred to as HTS materials. Even machines with this conductor type require appropriate expansion compensation in the axial direction due to the temperature differences between the operating temperature of the superconductor material and the external temperature of the hotter rotor outer housing.